1. Field of the Invention
The present invention relates to a III–V group nitride system semiconductor self-standing substrate and, in particular, to a III–V group nitride system semiconductor self-standing substrate with a uniform dislocation density distribution and a reduced dislocation on its surface, a method of making the same, and a III–V group nitride system semiconductor wafer with a nitride system semiconductor layer epitaxially grown on the substrate.
2. Description of the Related Art
Nitride system semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN) have a sufficiently wide bandgap and are of direct transition type in inter-band transition. Therefore, they are a great deal researched to be used for short-wavelength light emitting devices. Further, they have a high saturation drift velocity of electron and can use two-dimensional carrier gases in hetero junction. Therefore, they are also expected to be used for electronic devices.
Nitride semiconductor layers to compose the device are epitaxially grown on an underlying substrate by using a vapor-phase growth process such as MOVPE (metal organic vapor phase epitaxy), MBE (molecular beam epitaxy) and HVPE (hydride vapor phase epitaxy). However, since there is no underlying substrate that has a lattice constant matched to that of nitride semiconductor layers, it is impossible to obtain a high quality of grown layer and therefore a number of crystal defects (dislocations) are generated in the resulting nitride semiconductor layers. Since the crystal defects interfere with enhancement in device properties, researches to reduce the crystal defect in nitride semiconductor layers are a great deal conducted so far.
In order to obtain a III-group element nitride system crystal with relatively few crystal defects, a method is known that a low-temperature deposition buffer layer (buffer layer) is formed on a hetero-substrate such as sapphire and then an epitaxial layer is grown thereon. The crystal growth method using the buffer layer is conducted as follows. AlN or GaN is deposited on a sapphire substrate nearly at 500° C. to form an amorphous film or a continuous film including polycrystal. This film is heated to about 1000° C. and thereby part thereof is evaporated or crystallized to form crystal nuclei with a high density. Then, using the crystal nuclei as growth nuclei, a GaN film with a relatively good crystalline quality is grown. However, even when the method using the buffer layer is conducted, the resulting GaN film includes a considerable number of crystal defects such as a penetrating dislocation or a vacancy. Thus, it is not a satisfactory method for obtaining a high-performance device desired currently.
In order to solve the above problem, a method is a great deal researched that a GaN substrate is used as a crystal growth substrate and semiconductor layers to compose a device section are grown thereon. Herein, a GaN substrate for crystal growth is called GaN self-standing substrate. A known method of making the GaN self-standing substrate is ELO (epitaxial lateral overgrowth; e.g., Appl. Phys. Lett. 71 (18) 2638 (1997)). The ELO is conducted such that a mask with openings is formed on an underlying substrate and a GaN layer with reduced dislocations is laterally grown through the openings. For example, Japanese patent application laid-open No. 11-251253 discloses a method of making a self-standing GaN substrate that a GaN layer is grown on a sapphire substrate by ELO and then the sapphire substrate is removed by etching.
Further, FIELO (facet-initiated epitaxial lateral overgrowth; e.g., A. Usui et al., Jpn. J. Appl. Phys. Vol. 36(1997), pp. L899–L902) is known that is a modification of ELO. The FIELO is in common with the ELO in that a mask of silicon dioxide is used for selective growth, and is different from the ELO in that a facet is formed at the mask opening in the selective growth. By forming the facet, the propagation direction of dislocation is varied so as to reduce the penetrating dislocation to reach the upper surface of epitaxial growth layer. Using the FIELO, a thick GaN layer is grown on an underlying substrate such as sapphire. Then the underlying substrate is removed. Thus, a GaN self-standing substrate with relatively few crystal defects can be obtained.
Still further, DEEP (dislocation elimination by the epi-growth with inverted-pyramidal pits: e.g., K. Motoki et al., Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L140–L143, Japanese patent application laid-open No. 2003-165799) is known as a method for making a GaN self-standing substrate with few dislocations. The DEEP is conducted such that a mask of silicon nitride etc. patterned on a GaAs substrate is used in growing a GaN layer, where a plurality of pits are formed intentionally surrounded by facets on the surface of crystal to collect dislocations at the bottom region of pit, thereby reducing the number of dislocations at the other region.
In the abovementioned ELO and DEEP, the initial crystal growth is conducted while generating facets on the crystal growth interface. There is a property that the propagation direction of a dislocation propagating during the crystal growth is bent by the facet. Using the property, the dislocation density on the surface of substrate can be reduced while preventing the dislocation from reaching the upper surface of crystal. On the other hand, by generating the pit surrounded by facets at the crystal growth interface in crystal growth, dislocations are densely gathered at the bottom region of pit. Due to the collecting of dislocations, the dislocations can be eliminated while running into each other, or the penetration of dislocation toward the upper surface can be stopped while having a dislocation loop. Thereby, the dislocation density can be reduced effectively.
Furthermore, VAS (void-assisted separation: e.g., Japanese patent application laid-open No. 2003-178984) is known as a method for making a III group nitride system semiconductor substrate with few dislocations. The VAS is conducted such that a metal film is formed on an underlying substrate with a first III group nitride system semiconductor layer formed thereon or on an underlying substrate made of the first III group nitride system semiconductor, the underlying substrate is thermally treated in an atmosphere that contains hydrogen gas or hydrogen-contained compound gas to form voids in the first III group nitride system semiconductor layer, and then a second III group nitride system semiconductor layer is formed on the metal film.
Although a GaN substrate is obtained by growing a GaN layer by HVPE on a hetero-substrate based on ELO, DEEP etc. and then separating the GaN layer from the underlying substrate, such a GaN substrate has generally in its as-grown state a morphology that pits or hillocks are generated on the upper surface and its back side surface also has a rough surface like a frosted glass. Since it is difficult to grow there on an epitaxial layer for device fabrication if nothing is done, the upper and lower surfaces of the substrate are generally mirror-finished by polishing to use it for device fabrication.
In conventional semiconductor substrates such as Si and GaAs, there cannot occur a problem that dislocation density or its distribution is significantly dispersed on the upper and lower surfaces since the substrates can be made by using a method of cutting away a substrate from its crystal ingot.
However, in the case of GaN self-standing substrate, since thick GaN crystal epitaxially grown on a hetero-substrate is separated (peeled) from the hetero-substrate after the growth to obtain the GaN self-standing substrate, it is difficult to suppress the occurrence of dislocation near at the hetero-epitaxial growth interface corresponding to the initial stage of crystal growth. Therefore, the dislocations densely gathered need to be reduced during the growth of thick epi-crystal film to be used as the self-standing substrate so as to finally have a substrate surface with reduced dislocations. Accordingly, the abovementioned dislocation-reducing methods such as ELO, FIELO and DEEP are developed.
However, the distribution of dislocation density is highly dispersed on the substrate surface although the dislocation density can be reduced in the GaN self-standing substrate thus fabricated. Especially, it is frequently observed that a region with a high dislocation density is locally generated on the substrate surface.
If the crystal growth is continued while leaving facets of concaves generated on the crystal growth interface, the region with dislocations densely gathered always remains on the growth surface. In the DEEP abovementioned, which is a method for intentionally using the dislocation-reduced region generated between the regions with dislocations densely gathered, uniformity in dislocation density distribution on the entire substrate surface must be sacrificed although a region with significantly reduced dislocation density can be obtained.
Japanese patent application laid-open No. 2003-178984 discloses that the crystal growth interface is flattened by terminating the facet growth by increasing the amount of hydrogen mixed into carrier gas or by changing the crystal growth conditions in process of the crystal growth, and dislocations gathered at the bottom region of pit are dispersed again as the crystal growth goes on.
However, in conventional manner, the growth of substrate and the polishing are performed without considering the dislocation density distribution in a depth direction of substrate. Therefore, it was in fact impossible to obtain a substrate surface that gathered dislocations are perfectly dispersed, or a substrate surface obtainable when dislocations are uniformly dispersed was unintentionally removed by polishing at the substrate surface. As a result, the substrate surface being mirror-finished had frequently a highly dispersed distribution in dislocation density.
When such a region with dislocations densely gathered is left on the substrate surface, a device with epitaxial layers grown thereon must have a deteriorated characteristic. For example, the output of a laser diode will lower and the lifetime thereof will be shortened.